Abstract
Abstract. To predict atmospheric partitioning of organic compounds between gas and aerosol particle phase based on explicit models for gas phase chemistry, saturation vapor pressures of the compounds need to be estimated. Estimation methods based on functional group contributions require training sets of compounds with well-established saturation vapor pressures. However, vapor pressures of semivolatile and low-volatility organic molecules at atmospheric temperatures reported in the literature often differ by several orders of magnitude between measurement techniques. These discrepancies exceed the stated uncertainty of each technique which is generally reported to be smaller than a factor of 2. At present, there is no general reference technique for measuring saturation vapor pressures of atmospherically relevant compounds with low vapor pressures at atmospheric temperatures. To address this problem, we measured vapor pressures with different techniques over a wide temperature range for intercomparison and to establish a reliable training set. We determined saturation vapor pressures for the homologous series of polyethylene glycols (H − (O − CH2 − CH2)n − OH) for n = 3 to n = 8 ranging in vapor pressure at 298 K from 10−7 to 5×10−2 Pa and compare them with quantum chemistry calculations. Such a homologous series provides a reference set that covers several orders of magnitude in saturation vapor pressure, allowing a critical assessment of the lower limits of detection of vapor pressures for the different techniques as well as permitting the identification of potential sources of systematic error. Also, internal consistency within the series allows outlying data to be rejected more easily. Most of the measured vapor pressures agreed within the stated uncertainty range. Deviations mostly occurred for vapor pressure values approaching the lower detection limit of a technique. The good agreement between the measurement techniques (some of which are sensitive to the mass accommodation coefficient and some not) suggests that the mass accommodation coefficients of the studied compounds are close to unity. The quantum chemistry calculations were about 1 order of magnitude higher than the measurements. We find that extrapolation of vapor pressures from elevated to atmospheric temperatures is permissible over a range of about 100 K for these compounds, suggesting that measurements should be performed best at temperatures yielding the highest-accuracy data, allowing subsequent extrapolation to atmospheric temperatures.
Highlights
Atmospheric oxidation of organic vapors can lead to lowvolatility and semivolatile organic compounds (LVOCs and SVOCs), which are multifunctional in nature with molar masses typically between 150 and 300 g mol−1 and saturation vapor pressures between 0.1 and 10−7 Pa (Jimenez et al, 2009; O’Meara et al, 2014)
Pentaethylene glycol could be measured with all three techniques; some instruments were not able to measure the glycols with high saturation vapor pressures because of fast evaporation, while other instruments reached their lower limit of detection for glycols with larger molecular weight and low saturation vapor pressure
The measurements for PEG3 were performed at temperatures below room temperature and at relative humidities ranging from almost 0 % at the lowest temperatures to 94 % relative humidity at 288 K
Summary
Atmospheric oxidation of organic vapors can lead to lowvolatility and semivolatile organic compounds (LVOCs and SVOCs), which are multifunctional in nature with molar masses typically between 150 and 300 g mol−1 and saturation vapor pressures between 0.1 and 10−7 Pa (Jimenez et al, 2009; O’Meara et al, 2014) In this range of vapor pressures individual compounds partition dynamically between the gas and particle phases, depending on total aerosol loading and temperature. Even for the straight-chain dicarboxylic acids the experimental saturation vapor pressures reported in the literature deviate by up to four orders of magnitude between different measurement techniques (Bilde et al, 2015), and the difference can become as large as six orders of magnitude when additional functional groups are added to the straightchain dicarboxylic acids (Huisman et al, 2013) These differences are strikingly larger than the error estimates for the individual techniques, which are at most stated as a factor of 2. A very interesting observation of the Bilde et al (2015) study when comparing different data sets was “. . . that the temperature dependence of the saturation vapor pressure, i.e., the slopes of the individual data sets (the enthalpies of sublimation and vaporization), agree almost always better with each other than the reported saturation vapor pressures themselves.” Obviously, there are systematic biases of the different techniques, which are neither fully understood nor characterized
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